On Wed, Jun 11, 2008 at 11:39 PM, Levi Pearson <[EMAIL PROTECTED]> wrote:
snip > piston and/or cylinder apart or bending the rods. High-compression > gas engines are more efficient, but they also require high-octane fuel > to avoid pre-detonation. > > Turbocharging effectively increases the compression ratio, making > pre-detonation even more likely. Thus, timing has to be pulled and > more fuel added as boost increases beyond atmospheric pressure, which > dramatically reduces efficiency and power below what optimum spark > timing and fuel mixture would bring. This is why turbocharged gas > engines can be tuned to much higher performance on race gas than pump > gas. This is why all decent engine-management systems advance the timing of your ignition cycle when the turbo is making boost. Obviously there's a limit to how far you can advance your timing, but in order to be "safer than sorry", many production, stock engines actually fire slightly after top-dead-center. Advancing the timing as close as you can to top-dead-center slightly mitigates the problem. Timing advancement is _not_ a substitute for effective octane level. It merely helps a little. And you are correct, in that a turbo only increases efficiency when it is producing boost and the amount of fuel stays constant. Which is why my Passat got better mileage at 90+ mph (turbo was making boost but the valve timing was still in "cruising" mode) than at lower speeds. The combination of slight boost + slight timing advancement with (nearly)constant fuel consumption made it more efficient. That was one of the things I hated about that car. Because the powerband vs. boost-production curves weren't very well matched it got _horrible_ mileage in-town (usually below 20). Well, brethren, I believe we have beat this horse well beyond death and are bordering on the glue phase. -- Alex Esplin /* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
